style: enforce ISO-8601 in deprecation date

[Rylan12]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Add RuboCop check to make sure deprecate! date is ISO-8601 compliant

Corresponding homebrew-core PR: Homebrew/homebrew-core#56903

[sjackman]

Thank you, Rylan!

[bayandin]

Well done 👍

[Rylan12]

Thanks! Once the formulae are merged into linuxbrew-core I'll rerun the CI and (assuming it's green) merge this PR.

[reitermarkus]

Just a small nitpick: ISO 8601 instead of ISO-8601

[reitermarkus]

@Rylan12, the tests still use ISO-8601, would be great if you could also change them to ISO 8601.

[Rylan12]

@reitermarkus Ahh, you're right. Looks like I jumped the gun a little on this one. Apologies for that.

Can I commit these directly or should I still make a PR?

[reitermarkus]

Always create a PR so that CI can run on it before merging.

[Rylan12]

Makes sense. I'll cc you on the PR. Thanks!

[MikeMcQuaid]

Nice work @Rylan12!